Process for producing heat-resistant intermetallic compound Ni3Al foil having room-temperature ductility and heat-resistant intermetallic compound Ni3Al foil having room-temperature ductility

ABSTRACT

A process for producing a heat-resistant intermetallic compound Ni 3 Al foil having a room-temperature ductility, which comprises a first step of arc-melting an alloy having a chemical composition containing Ni as a main component and Al to form a starting rod, a second step of growing the starting rod in columnar crystal form by unidirectional solidification, a third step of cutting out the unidirectionally solidified rod to form a plate, and a fourth step of cold-rolling the plate cut at room temperature to form a foil. The invention can provide a process for producing a thin Ni 3 Al foil which has a thickness of 200 microns or less and which is excellent in high-temperature strength, oxidation and corrosion resistances and room-temperature ductility.

FIELD OF THE INVENTION

The present invention relates to a process for producing aheat-resistant Ni₃Al intermetallic compound having an excellentroom-temperature ductility. More specifically, it relates to a processfor producing a thin Ni₃Al foil which has a thickness of 200 microns orless and which is excellent in high-temperature strength, oxidation andcorrosion resistances, and room-temperature ductility.

DESCRIPTION OF THE RELATED ART

With respect to the improvement of the brittleness of Ni₃Al as aheat-resistant structural material, a method of adding trace boron whichwas studied in Tohoku University in 1979 is known. In the past, therewas an example of forming a plate having a thickness of approximately500 microns. However, an actual problem that brittleness tends to occurat a high temperature even with the addition of boron still remainedunresolved. Further, Ni₃Al containing trace boron was not suited forproduction of a foil because it had a low room-temperature tensileelongation of approximately 30% and a high yield stress and isconsequently low in rolling moldability.

Afterwards, Oak Ridge National Laboratory also studied the improvementof the alloy composition for reducing this material to practical use.

However, in any of the approaches in the past, the problem was to usethe material in the form of a plate, a block or a wire, and no attemptwas made to use it for production of a foil.

Unlike ordinary metallic materials, the strength of Ni₃Al increases withincreasing temperature. At a temperature of 800° C. (or 1,073 K), thestrength reaches five times as high as that at room temperature as shownin FIG. 2. For this reason, the material has been considered to be usedas a heat-resistant material, in turbine blades, boiler pipes, fuelcladding pipes for a reactor and aerospace materials.

However, conventionally melt and cast Ni₃Al are so brittle as to bebroken soon after yielding. Such brittleness in the vicinity of roomtemperature which is common to intermetallic compounds.

Accordingly, it is difficult to produce a foil having a thickness of 200microns or less by cold rolling. It has been so far considered quitedifficult, and has not yet been realized.

SUMMARY OF THE INVENTION

Under these circumstances, the invention has been made. The objective ofthe invention is to provide a process in which a thin Ni₃Al foil havinga thickness of 200 microns or less can be produced by cold rolling froman Ni₃Al intermetallic compound excellent in high-temperature strength,oxidation and corrosion resistances, and high ductility.

Another objective of the invention is that the foil is applicable to theproduction of lightweight, heat-resistant structural materials such as ahoneycomb structure and a laminated complex material and many otherfunctional materials.

In order to accomplish the foregoing and other objectives, the inventionfirst provides a process for producing a heat-resistant intermetalliccompound Ni₃Al foil having a room-temperature ductility, which comprisesa first step of forming a stating rod of an alloy having a chemicalcomposition containing Ni as a main component and Al by arc-melting, asecond step of growing the starting rod in columnar crystal form byunidirectional solidification, a third step of cutting theunidirectionally solidified rod to form a plate, and a fourth step ofcold-rolling the plate at room temperature to form a foil.

The invention second provides the process for producing theheat-resistant intermetallic compound Ni₃Al foil, wherein the alloy inthe first step contains Al in an amount of at least 12.8% by weight andat most 13.6% by weight and has an L1₂-type ordered structure. Theinvention third provides the process for producing the heat-resistantintermetallic compound Ni₃Al foil, wherein the alloy in the first stepcontains a third element other than Al. The invention fourth providesthe process for producing the heat-resistant intermetallic compoundNi₃Al foil, wherein in the first step a rod having a diameter of 50 mmor less is formed as the starting rod. The invention fifth provides theprocess for producing the heat-resistant intermetallic compound Ni₃Alfoil, wherein the rate of unidirectional solidification in the secondstep is 25 mm/h or less. The invention sixth provides the process forproducing the heat-resistant intermetallic compound Ni₃Al foil, whereinin the third step, the thickness of the plate is 5 mm or less. Theinvention seventh provides the process for producing the heat-resistantintermetallic compound Ni₃Al foil, wherein in the cold-rolling of theplate in the fourth step, annealing is conducted at a temperature of800° C. (or 1,073 K) or more for 20 minutes or more. The inventioneighth provides the process for producing the heat-resistantintermetallic compound Ni₃Al foil, wherein after the fourth step, thework-hardened, cold-rolled foil is annealed with a degree of vacuum ofhigher than 10⁻³ Pa at a temperature of 800° C. (or 1,073 K) or more for20 minutes or more, and further cold-rolled to form a foil.

Moreover, the invention ninth provides a heat-resistant intermetalliccompound Ni₃Al foil having a room-temperature ductility which foil has achemical composition containing Ni as a main component and Al in anamount of at least 12.8% by weight and at most 13.6% by weight, and hasa thickness of 200 microns or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing dependence of strength of Ni₃Al ontemperatures.

FIG. 2 is an optical micrograph showing a columnar crystal structure ofunidirectionally solidified Ni₃Al.

FIG. 3 is a graph showing a tensile stress-strain curve ofunidirectionally solidified Ni₃Al.

FIG. 4 is an optical micrograph of a recrystallized structure obtainedby annealing a cold-rolled foil having a thickness of 70 microns at1,273 K for 30 minutes.

FIG. 5 is a view showing the result of bending test of therecrystallized sample at room temperature.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described in more detail below.

To begin with, the process for producing the Ni₃Al foil in the inventioncomprises a first step of arc-melting an alloy having a chemicalcomposition containing Ni as a main component and Al to form a startingrod, a second step of growing the starting rod in columnar crystal formby unidirectional solidification, a third step of cutting theunidirectionally solidified rod by electric discharge machining to forma plate, and a fourth step of cold-rolling the plate at room temperatureto form a foil. This process consequently makes it possible to producethe thin Ni₃Al foil which has the thickness of 200 microns or less andwhich is excellent in high-temperature strength, oxidation and corrosionresistances and room-temperature ductility.

The arc-melting in the first step is preferably conducted in an inertgas such as argon or helium. The starting rod formed by this arc-meltingis not particularly limited. For unidirectional solidification in thesecond step, for example, a rod having a diameter of 50 mm or less and alength of 300 mm or less is considered.

The formation of the unidirectionally solidified rod in the second stepcan preferably be conducted by, for example, a floating zone method in alight image furnace. In this case, an appropriate rate of unidirectionalsolidification is 25 mm/h or less. When the rate exceeds this value, nouniform unidirectionally solidified rod is obtained.

The production of the unidirectionally solidified rod is quite importantin the process of the invention. This is because the unidirectionalsolidification imparts such a very excellent rolling formability that ayield stress is low and a tensile elongation is 60% or more to the Ni₃Alintermetallic compound without adding other alloy elements.

The formation of the plate in the third step is conducted by cutting theunidirectionally solidified rod obtained in the second step through, forexample, using electric discharge machining. At this time, the size ofthe plate is not particularly limited. For smoothly conducting thesubsequent fourth step, it is advisable that the thickness is 5 mm orless.

In the fourth step, the cold rolling is conducted at room temperature.The annealing may be conducted, as required, at a temperature of 800° C.(or 1,073 K) or more for 20 minutes or more. Further, the annealing andthe cold rolling may be repeated several times. In this repeating cycle,it is advisable that the annealing may be conducted, for example, undera degree of vacuum of higher than 10⁻³ Pa at a temperature of 800° C.(or 1,073 K) or more for 20 minutes or more.

The invention is illustrated more specifically by referring to thefollowing Example.

EXAMPLE

First, an alloy having a chemical composition containing Ni as a maincomponent and 12.8 to 13.6% by weight of Al was arc-melted in ahigh-purity argon gas to form a starting rod having a diameter ofapproximately 15 mm and a length of approximately 150 mm (first step).

In this case, additives such as manganese, iron and boron can also beadded.

It was identified that this Ni₃Al intermetallic compound had atemperature dependence of strength which, unlike ordinary metals,increased in strength with increasing temperatures up to 900 K in eachof the crystal orientations A, B and C.

Subsequently, a part of this starting rod was unidirectionallymelt-solidified in a light image furnace with halogen lamps as a lightsource by a floating zone method (second step).

In this case, the rate of unidirectional solidification was 25 mm/h orless. Consequently, it was identified, as shown in FIG. 2, that acolumnar crystal of Ni₃Al in a single phase was developed in a growthdirection.

A product having quite a high tensile ductility of 60% or more as shownin FIG. 3 is obtained by controlling the solidification to make uniformthe columnar crystal structure.

Subsequently, the unidirectionally solidified rod was cut into a platehaving a thickness of 1 mm, a width of 7 mm and a length of 70 mm byelectric discharge machining-(third step), and the plate was thencold-rolled at room temperature (fourth step). In this case, the platecould be rolled into a foil having a thickness of 50 microns with areduction of 95% without intermediate annealing. In this step, the platemay be annealed, as required, at a temperature of 800° C. (or 1,073 K)or more for 20 minutes or more.

The cold-rolled foil was work-hardened, and hardly deformed in the nextstep. However, when the annealing was conducted with a degree of vacuumof higher than 10⁻³ Pa at a temperature of 800° C. (or 1,073 K) or morefor 20 minutes or more, even a recrystallized foil having a structureshown in FIG. 4 (recrystallized structure obtained by annealing acold-rolled foil having a thickness of 70 microns for 30 minutes at1,273 K), for example, could show a great ductility. When this foil wasfurther cold-rolled (fifth step), a thinner foil could be formed.

Lightweight, heat-resistant structures such as a honeycomb structure anda laminated complex material can be produced by using the foil which isobtained by the process of the invention. The lightweight,heat-resistant structures can be used in power generation turbine,aerospace heat-shielding materials and engine members, and automobileengines.

Further, they are also used in heating elements and shielding materialsunder strongly corrosive environment by utilizing the oxidation andcorrosion resistances, and in various fields.

Such a ductile, heat-resistant foil alone is used in variousapplications. When it is used in combination with other materials, muchmore applications are considered.

In this Example, the results of the unidirectional solidification by thefloating zone method and the cold rolling are demonstrated. Anotherprocess comprising a combination of unidirectional solidification andhot rolling capable of producing Ni₃Al columnar crystal can also beincluded.

According to the invention, the thin foil can be produced from thebrittle intermetallic compound by cold rolling, and the characteristicsof the intermetallic compound which was hardly put to practical usebecause of the brittleness can effectively be utilized.

Further, such a foil can be used to produce lightweight, heat-resistantstructural materials such as a honeycomb structure and a laminatedcomplex material and many other functional materials. Thus,general-purpose materials can be provided.

Moreover, when the lightweight, heat-resistant structures can beproduced, the development of high-performance power generation turbines,high-performance jet engines and space vehicles is promoted, andeconomical effects and effects of global environmental protection suchas inhibition of exhaust carbon dioxide gas can be expected.

1-8. (canceled)
 9. A heat-resistant intermetallic compound Ni₃Al foilhaving a room-temperature ductility which foil has a chemicalcomposition containing Ni as a main component and Al in an amount of atleast 12.8% by weight and at most 13.6% by weight, and has a thicknessof 200 microns or less.